Move reference extraction logic from the German bank abstract class to the general abstract class

[beilsma]

The logic for getting the references from the statement line (:61:) was defined in the GermanBank class, which German bank parsers extend.

Since Dutch banks use the identical statement line structure, this same logic can be used to extract these references from their statements as well.

This means the logic can be moved from the GermanBank class to the AbstractParser class, which is the base for all bank parsers, both German and Dutch banks.